Time-of-flight radio location system

ABSTRACT

A bi-static radar configuration measures the direct time-of-flight of a transmitted RF pulse and is capable of measuring this time-of-flight with a jitter on the order of about one pico-second, or about 0.01 inch of free space distance for an electromagnetic pulse over a range of about one to ten feet. A transmitter transmits a sequence of electromagnetic pulses in response to a transmit timing signal, and a receiver samples the sequence of electromagnetic pulses with controlled timing in response to a receive timing signal, and generates a sample signal in response to the samples. A timing circuit supplies the transmit timing signal to the transmitter and supplies the receive timing signal to the receiver. The receive timing signal causes the receiver to sample the sequence of electromagnetic pulses such that the time between transmission of pulses in the sequence and sampling by the receiver sweeps over a range of delays. The receive timing signal sweeps over the range of delays in a sweep cycle such that pulses in the sequence are sampled at the pulse repetition rate, and with different delays in the range of delays to produce a sample signal representing magnitude of a received pulse in equivalent time. Automatic gain control circuitry in the receiver controls the magnitude of the equivalent time sample signal. A signal processor analyzes the sample signal to indicate the time-of-flight of the electromagnetic pulses in the sequence.

The United States Government has rights in this invention pursuant toContract Number W-7405-ENG-48 between the United States Department ofEnergy and the University of California for the operation of LawrenceLivermore National Laboratory.

RELATED APPLICATIONS

This application is a continuation-in-part (CIP) of Ser. No. 08/058,398,filed May 7, 1993, which is a continuation-in--part (CIP) of Ser. No.08/044,745, filed Apr. 12, 1993, now U.S. Pat. No. 5,343,471, issuedSep. 6, 1994.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to high resolution position measurementsystems, and more particularly to sub-millimeter resolutiontime-of-flight radio location systems operating over a range of lessthan about ten feet.

2. Description of Related Art

There are a wide variety of position-sensing technologies availablewhich rely on non-mechanical technologies, such as the following:

(1) Infrared or optical systems that employ TV cameras and complex videotracking algorithms;

(2) Ultrasound systems; and

(3) Continuous wave (CW) radio frequency fringe counting and phasemeasurements systems.

The infrared or optical systems are very expensive and, therefore, areimpractical for most commercial applications of position sensing. Theultrasound systems are quite inaccurate and unreliable. The CW radiofrequency fringe counting and phase measurement systems suffer severemulti-path problems and ambiguities in the position-sensing data. Noneof these prior systems provide high resolution, and all are quiteexpensive.

High resolution, low-cost position-sensing can be applied to theinteractive media arts, robotics, automotive occupant position sensing,digital surgery and a wide variety of applications where high resolutionposition sensing is desired.

SUMMARY OF THE INVENTION

The present invention is based on a bi-static radar configuration whichmeasures the direct time-of-flight of a transmitted RF pulse. Thissystem is capable of measuring time-of-flight with a jitter on the orderof about one pico-second, or about 0.01 inch of free space distance foran electromagnetic pulse, over a range of about one to ten feet or more.The system can be implemented with very low-cost components, so it canbe used in high-volume computer games and virtual reality systems.

The invention can be characterized as an apparatus for measuringtime-of-flight of an electromagnetic pulse. This apparatus is comprisedof a transmitter, which transmits a sequence of electromagnetic pulsesin response to a transmit timing signal, and a receiver, which samplesthe sequence of electromagnetic pulses with controlled timing inresponse to a receive timing signal and generates a sample signal inresponse to the samples. A timing circuit supplies the transmit timingsignal to the transmitter and supplies the receive timing signal to thereceiver. The receive timing signal causes the receiver to sample thesequence of electromagnetic pulses such that the time betweentransmission of pulses in the sequence and sampling by the receiversweeps over a range of delays. A sample signal produced by the detectioncircuitry indicates the time-of-flight between the transmitter and thereceiver of pulses in the sequence in response to the sample signal andthe timing circuit. The receive timing signal sweeps over the range ofdelays in a sweep cycle such that pulses in the sequence are sampled atthe pulse repetition rate, and with different delays in the range ofdelays to produce a sample signal representing magnitude of a receivedpulse in equivalent time. Automatic gain control circuitry in thereceiver controls the magnitude of the equivalent time sample signal. Asignal processor analyzes the sample signal to indicate thetime-of-flight of the electromagnetic pulses in the sequence. Forinstance, the system may include a pulse detection signal whichgenerates a pulse detect signal in response to the sample signal whenthe sample signal reaches a threshold during a sweep over the range ofdelays. The time between beginning of a sweep and generation of thepulse detect signal indicates the time-of-flight of the pulses in thesequence.

In one aspect of the invention, the timing circuit includes a pulse-rateclock, a cable, having a known cable delay time connecting thepulse-rate clock to the transmitter, and a circuit in the transmitterwhich translates the pulse-rate clock from the cable into the transmittiming signal. A controlled delay circuit coupled with the pulse-rateclock produces the receive timing signal at the receiver in response tothe pulse-rate clock, delayed in time to compensate for the cable delaytime.

The receive timing signal can be produced using a controllable delaycircuit that includes a voltage controlled delay circuit. A voltage rampgenerator coupled to the control input of the voltage controlled delaycircuit causes a sweep in the delay of the receive timing signal. Thevoltage ramp generator may be implemented using a digital-to-analogconverter, or using analog ramp generators. In one embodiment, an analogramp generator is used, which has an exponential transfercharacteristic, and the voltage controlled delay circuit implements acomplimentary exponential delay in response to the control input toprovide a nearly linear delay over the range of delays.

The system may include a plurality of receivers spaced away from oneanother, such that the position of the transmitter can be detected withseveral degrees of freedom.

The invention can also be characterized as a method for detecting theposition of an object at a range of less than about ten feet. The methodis comprised of the following steps:

(1) mounting a transmitter on the object;

(2) transmitting from the transmitter a sequence of electromagneticpulses;

(3) detecting time-of-flight of the electromagnetic pulses from thetransmitter to the receiver; and

(4) processing the time-of-flight to indicate the position of theobject.

The step of detecting time-of-flight includes sampling the sequence ofpulses with controlled timing to produce an equivalent timerepresentation of a transmitted pulse at the receiver, and processingthe equivalent time signal to indicate the time-of-flight.

As mentioned above, the present invention operates over a range oftime-of-flights of less than ten nanoseconds with excellent accuracy.The system according to the present invention can be implemented withsub-millimeter sensitivity over a range of less than ten feet.

Other aspects and advantages of the present invention can be seen uponreview of the figures, the detailed description and the claims whichfollow.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a simplified diagram of a position sensor based on directtime-of-flight measurements of electromagnetic pulses according to thepresent invention.

FIG. 2 is a block diagram of the position sensor according to thepresent invention.

FIG. 3 is a more detailed functional block diagram of a receiver andtransmitter in a position detector according to the present invention.

FIG. 4 is an electrical schematic diagram of a timing circuit and otherparts of a position sensor according to the present invention.

FIG. 5 is an electrical schematic diagram of a transmitter for aposition sensor according to the present invention.

FIG. 6 is an electrical schematic diagram of a receiver for a positionsensor according to the present invention.

FIG. 7 is a simplified diagram of a head position sensing systemimplemented according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A detailed description of preferred embodiments of the present inventionis provided with reference to the figures, in which FIG. 1 provides anillustration of the basic concept. In particular, a position sensoraccording to the present invention includes a transmitter 10 which ismounted on an object 11, the position of which is to be sensed. Thetransmitter 10 generates a sequence of RF pulses 12 in response to atransmit timing signal supplied across a timing cable 13 from a controlcircuit 14. The receiver 15 coupled to the control circuit 14 includes asample circuit which samples the RF pulse with controlled timing inresponse to a receive timing signal. The sensor control circuit 14supplies the receive timing signal through a controlled delay circuit 16to the receiver 15 so that the sequence of electromagnetic pulses aresampled with a time between transmitting of pulses from the transmitter10 and sampling by the receiver 15 precisely controlled and swept over arange of delays. The receiver generates a sample signal on line 17,which is supplied to the sensor control circuit 14 which detects acharacteristic of the sample signal to indicate a time-of-flight of theRF pulses 12 from the transmitter 10 to the receiver 15. In the systemillustrated in FIG. 1, a first additional receiver 18 and a secondadditional receiver 19 are included in the system. The receivers 18 and19 are connected to the control circuit 14 by timing cables 20 and 21,respectively, and also sample the RF pulses 12 with controlled timing.The time-of-flight detected in response to the receivers 15, 18 and 19can be processed to indicate the position of the object 11 with a numberof degrees of freedom and with excellent resolution according to thepresent invention. Also the range indicated by bracket 22 between thetransmitter 10 and the receivers 15, 18, 19 may be less than 10 feet.

The operation of the sensor according to the present invention can bebetter understood with respect to the block diagram in FIG. 2. Thesystem is based on a pulse timing oscillator 50 which generates a clockat a pulse repetition rate of about two megahertz in the example to beillustrated with respect to FIG. 2. The pulse rate clock generated bythe pulse timing oscillator is supplied across a cable 51 to a pulsegenerator 52 in the transmitter. The pulse generator generates an RFpulse, such as centered at nominally 2.0 gigahertz, which is transmittedby the transmitter 53 through antenna 54 with the pulse repetition rateof about two megahertz.

The pulse timing oscillator 50 is also coupled to a controlled delaycircuit 55. The controlled delay circuit 55 supplies a receive timingsignal on line 56 to a receive strobe generator 57. The receive strobegenerator 57 strobes a sample gate in the receiver 58 at the pulserepetition rate, but at times which are delayed relative to the timethat the transmitter emits the RF pulse.

The controlled delay circuit 55 is controlled by a start/stop circuit 59which includes a sweep oscillator which oscillates at about 70 Hertz inthe example described. Thus, the sweep oscillator is designed tooscillate at less than 100 Hertz. An alternative system may beimplemented which sweeps at about 16 kilohertz to be compatible withNTSC video. This oscillator supplies a ramp signal on line 60 to theswept delay circuit 55 to control the timing of the strobes generated bythe strobe generator 57. The start/stop circuit 59 generates a startsignal at the beginning of each sweep, and a stop signal in response toa pulse detect signal on line 61. The pulse detect signal on line 61 isgenerated by the receiver 58 in response to the samples of the sequenceof RF pulses. When the receiver 58 detects the RF pulse, the pulsedetect signal on line 61 causes the start/stop circuit 59 to issue astop signal. The start signal is used to initiate counter 62 and thestop signal is used to stop the counter 62. The counter begins countingat a count rate of about 10 megahertz in the example illustrated at thebeginning of each sweep, and stops counting upon receiving the stopsignal. Thus, the value in the counter 62 upon receiving the stop signalindicates the time-of-flight of an RF pulse from the transmitter to thereceiver (the difference between the delay of a strobe pulse at thebeginning of a sweep and the delay of a strobe pulse when the pulsedetect signal is generated). This value is supplied to processor 63which uses the information to determine the position of the transmitter53.

Thus, in the example illustrated, with a 70Hertz start/stop circuit, aramp which lasts about 14 milliseconds is produced. The delay betweenthe transmission of an RF pulse by the transmitter, and strobing of thereceiver by the receive strobe generator, sweeps over a range inresponse to the ramp. With a two megahertz pulse repetition rate, thetime between pulses is about 500 nanoseconds. The delay betweentransmission by the transmitter and reception by the receiver for a10-foot propagation would be about 10 nanoseconds. Thus, the timing ofthe receive strobe generated by the receive strobe generator can beswept over a range of delays which begin at a time compensating forcable delay to the transmitter and varies by about 10 nanoseconds inorder to precisely detect the position of the transmitter. With a ramplasting 14 milliseconds, and a pulse repetition rate of two megahertz,the receiver will sample about 28,600 pulses per sweep of the delaycircuit 55.

As illustrated in FIG. 2, the RF pulses at the receiver may have anamplitude which varies as illustrated by trace 70. The 2.0 gigahertzpulse generator will have a cycle time of about 500 picoseconds, with arise time on the leading edge of less than 100 picoseconds. The leadingedge of the pulse will appear as a strong pulse as indicated at point71. The strong pulse will be followed by a noise region, generally 72,which is based on reflections and other effects of the transmittedpulse. By the time the second pulse is generated, the noise will havedied to a low level as indicated by the region 73 at the beginning of areal-time pulse cycle.

This real-time pulse will be sampled over a range of delay times, suchthat an equivalent time signal, as illustrated at trace 80, is produced.This trace will assume the shape of the average pulse, however, with arepetition cycle of 70 Hertz for an equivalent time sample width of 14milliseconds. The equivalent cycle time of the pulse will be about 700microseconds, for an equivalent time pulse frequency of about 1.4kilohertz. Thus, the receiver includes an audio-frequency amplifier withautomatic gain control, and a threshold detector. Upon detection of thethreshold, such as indicated at point 81, the pulse detection signal isgenerated on line 61.

Also as illustrated in FIG. 2, the pulse timing oscillator 50 can beconnected to other transmitters, such as across cables 85 and otherreceivers, such as across cables 86 to produce a sophisticatedposition-detecting system.

The pulse timing oscillator 50 can be frequency modulated, or dithered,for the purpose of reducing interference from similar systems, or otherRF devices.

FIG. 3 provides a more detailed schematic diagram of a time-of-flightsensor according to the present invention. In the embodiment of FIG. 3,the pulse repetition frequency of about 2.5 megahertz is shown, which isgenerated by a 10 megahertz clock 100 connected to a divide-by-fourcircuit 101. The output of the divide-by-four circuit 101 provides apulse rate clock on line 102. This signal is supplied to a driver 103coupled to a timing cable 104. The timing cable is connected to thetransmit unit, which includes a driver 105, a pulse-forming network 106which is responsive to the driver 105, and a microwave oscillator 107which generates, for example, a two gigahertz, gated RF pulse on line108. Line 108 is coupled to a transmit antenna 109 which transmits theRF pulse 110 to a receive antenna 111.

The pulse timing signal on line 102, is supplied as a trigger signal toa voltage controlled delay circuit 112. The output of the voltagecontrolled delay circuit on line 113 drives a pulse-forming network 114which is used to strobe a sample-and-hold gate 115, which is connectedto receive antenna 111 and supplies a sample of the transmitted pulse toa holding capacitance 116. The holding capacitance drives an amplifier117 to produce a sample signal on line 118. The sample signal issupplied to a comparator 119 which compares the sample signal against athreshold 120. Also, the sample signal line 118 drives a peak-detectcircuit 139 which is based on a diode 120 and capacitor 121. The outputof the peak-detect circuit is supplied to an amplifier 122 whichprovides automatic gain control to the amplifier 117. The automatic gaincontrol amp includes an input resistor 123 which is connected to thefirst input of a differential amplifier 124. The second input ofdifferential amplifier 124 is connected to a reference voltage 125.Capacitor 126 is connected in feedback across the amp 124.

The voltage controlled delay circuit is controlled in response to a rampsignal on line 127, which is supplied by ramp generator 128. The rampsignal on line 127 causes the voltage controlled delay to sweep over arange of delays which corresponds to the delay of the timing cable 104plus a range of expected time-of-flights from the transmit unit to thereceiver. This ramp generator 128 is driven by a ramp clock 130. Theramp clock in the example illustrated is generated by dividing the pulserepetition frequency on line 102 by 2¹⁶ to produce a 40 Hertz signal online 131. The 40-Hertz signal is coupled to a binary storage element132. On the leading edge of the ramp clock, the output of the binary 132is set high, enabling the AND gate 133 to supply the 10 megahertz clockto a range counter 134. The range counter counts up at the 10 megahertzrate until the comparator 119 detects that the sample signal on line 118exceeds the threshold. At that point, the binary storage element 132 isreset, disabling the AND gate 133 and turning off the range counter 134.The data in the range counter can then be supplied out across bus 135 tothe signal processor. Also, on the leading edge of each 40 Hertz ramp, areset signal is supplied to control circuit 136 which resets the counter134 for a subsequent sweep. Thus, in FIG. 3, a 2.5 megahertz repetitionfrequency is derived from the 10 megahertz clock. The pulse repetitionfrequency drives the transmit unit through the timing cable, which maybe implemented with low-cost phonocables that carry DC power as well asthe clock. The transmitter comprises a pulse forming network (PFN) thatmodulates a gated RF oscillator to generate one cycle of RF as shown at,for example, two gigahertz center frequency. The RF monocycle propagatesfrom the transmit antenna to the receive antenna. A sample hold circuitin the receive unit samples the receive signal when driven with a gatepulse derived from the voltage controlled delay circuit and apulse-forming network. The hold circuit output is amplified by anautomatic gain controlled amplifier and applied to a thresholdcomparator. The output of the amplifier 117 is an equivalent timereplica of the RF pulse that repeats at a 40 Hertz rate, the sweep rateof the ramp generator.

A peak detector detects the maximum pulse amplitude in the equivalenttime sample signal and drives an automatic gain control amplifier tomaintain the peak amplitude of the equivalent time pulse at a controlledlevel, typically -1 peak volts in this example. The comparator istypically set to -0.5 volts to detect the equivalent time pulse at aprecise, constant percentage with a maximum level regardless offluctuation caused by time-of-flight range or antenna orientation.

The equivalent time signal represents a range sweep from one to ten feetas defined by the ramp circuit and the voltage controlled delay circuit.When the ramp starts its sweep, a binary is toggled to start the rangecounter by gating the 10 megahertz clock into the range counter. At thepoint in the sweep where the equivalent time pulse is preciselydetected, the range counter is stopped, leaving the exact range countfor readout. This cycle is repeated at a 25 millisecond rate. With a 10megahertz count rate, 250,000 counts represent full scale, or ten feet,so the digital resolution is in the neighborhood of 0.0006 inches.However, present systems are analog-noise limited to about 0.01 inchesat the 25 millisecond update rate. Digital averaging may be employed todecrease jitter.

The voltage controlled delay circuit 112 generates a linear-range sweepover time by employing a primitive exponential, high-speed voltage rampwith the time constant of about 10 nanoseconds. This ramp is combinedwith a primitive exponential ramp in the ramp circuit. Both thereal-time ramp and the equivalent-time ramp operate over the sameportions of their curves to jointly provide a linear sweep.

A representative electrical schematic diagram of another example of areceiver, transmitter and timing circuit are illustrated in FIGS. 4through 6. FIG. 4 illustrates the timing circuit for the systemaccording to the present invention.

The timing circuit includes a 70 Hertz oscillator, which is formed usingNAND gates 200 and 201 which have their inputs connected together tooperate as inverters. The first NAND gate 200 has its output connectedto the input of the second NAND gate 201. The second NAND gate 201 isconnected through a capacitor 202 in feedback to the input of the firstNAND gate 200. Also, the output of NAND gate 200 is connected throughresistor 203 to its input. This 70 Hertz clock is connected to aflip-flop through the RC differentiator composed of capacitor 204 andresistor 205, which is connected to the positive five-volt supply. Theflip-flop is based on NAND gate 206 and NAND gate 207. The output NANDgate 206 is connected to one input of NAND gate 207. The output of NANDgate 207 is connected to one input of NAND gate 206. The first input ofNAND gate 206 is the output of the RC circuit based on capacitor 204 andresistor 205. The second input to NAND gate 207 is the output of athreshold detection circuit at node 208. The threshold detection circuitat node 208 is composed of resistor 209, which is connected to receivethe sample signal, RX, generated by the receiver described in FIG. 6,and resistor 210, which is connected to the positive five-volt supply.

The output of the flip-flop composed of NAND gates 206 and 207 issupplied through resistor 211 to connector 212. The connector 212 drivesthe signal processor, which is composed of counter 213 and memory 214. A10 megahertz clock 215 drives a NAND gate 216. The second input to theNAND gate 216 is the signal from connector 212, which enables anddisables the output of the NAND gate 216 to drive the counter 213. Also,the signal from connector 212 is used as a read strobe for the memory online 215, and a reset signal on line 216 for resetting the counter byappropriate control circuitry. Thus, the counter is enabled at thebeginning of each cycle of the oscillator composed of NAND gates 200 and201, and turned off when the received sample signal RX falls below anegative threshold.

The system in FIG. 4 also shows the pulse clock composed of inverter 220and inverter 221. The output of inverter 220 is connected throughresistor 222 to its input. The output of inverter 221 is connectedthrough capacitor 223 to the input of inverter 220. The output ofinverter 221 is a 2 megahertz clock on line 224. This signal is suppliedthrough inverter 225 to node 226. Capacitor 227 is connected betweennode 226 and ground. Capacitor 228 is connected between node 226 and atransmit cable 229 such as an RCA phono plug coupled to a 12-footcoaxial audio-cable. Also, inductor 230 is coupled from cable 229 to thepositive five-volt supply in order to supply power to the transmitteracross the cable 229 superimposed with the transmit clock.

The pulse clock on line 224 is also supplied to a delay circuit which isconnected to line 224 through resistor 231. Resistor 231 is connected tothe input of inverter 232 and through variable capacitor 233 to ground.The variable capacitor 233 provides a coarse delay for the pulse clock.The output of inverter 232 is supplied through resistor 234 to the inputof inverter 235. The input of inverter 235 is also driven by the rampgenerator, generally 246. The output of inverter 235 is supplied throughinverter 236 to the receive strobe generator through capacitor 237.Capacitor 237 is connected between the output of inverter 236 and node238. Diode 239 has its anode connected to node 238 and its cathodeconnected to ground. Resistor 240 is connected between node 238 and thefive-volt supply. Node 238 is also connected to the emitter ofhigh-speed transistor 241. The base of transistor 241 is coupled toground. The collector of transistor 241 supplies the strobe signal STBon line 242. Also, the collector of transistor 241 is connected throughresistor 243 to the positive five-volt supply.

The ramp generator 246 is basically an analog exponential rampgenerator. This ramp generator may be replaced by a digital-to-analogconverter which digitally supplies a sequence of analog values to theinput of inverter 235 to control the delay using synchronousoscillators. In the analog version illustrated, the 70 Hertz clock atthe output of the NAND gate 200 is supplied on line 250 through resistor251 and the capacitor 252 to the base of transistor 253. Also resistor254 is connected from the base of transistor 253 to ground. Emitter oftransistor 253 is coupled to ground. The collector of transistor 253 isconnected through resistor 255 to the positive five-volt supply. Alsothe collector is coupled through capacitor 256 to ground. Resistor 257is connected from the positive five-volt supply to the input of inverter235. Resistor 258 is connected from the collector of transistor 253 tothe input of inverter 235. Also, controllable capacitor 259 is connectedfrom the input of inverter 235 to ground. This circuit serves to biasthe input of inverter 235 to a region in which it has an exponentialtransfer function. The ramp generator 246 generates a complimentaryexponential transfer function to provide overall a linear ramp in delayat the output of inverter 235. Fine control over the span of the rangeof delays produced is provided by the adjustable capacitor 259.

Also illustrated in FIG. 4 is a battery-based power supply. The powersupply includes a battery 270. The battery is connected to a switch 271which drives level translators 272 and 273 to provide isolated five-voltsupplies for the circuit.

Although the transmit timing signal and the receive timing signal areproduced using a single clock in the embodiment described, alternativesystems may employ timing circuits which have separate synchronizedclocks located at the receiver and transmitter, respectively, without acable tether.

FIG. 5 illustrates the implementation of a transmitter according to thepresent invention. This transmitter includes a connector 300 which canbe connected to the cable 229 illustrated in FIG. 4. This cable suppliesDC power and the transmit clock to a pulse-forming network in thetransmitter. Thus, connector 300 is coupled through inductor 301 to line302. Also, the connector 300 is connected through capacitor 303 to node304. Resistor 305 is connected from node 304 to line 302. Diode 306 hasits cathode coupled to node 304 and its anode coupled to ground. Diode307 has its anode coupled to node 304 and its cathode coupled to line302. Line 302 is coupled through capacitor 308 to ground. Node 304 iscoupled through inverters 309 and 310 in series which shape the incomingsignal. The output of inverter 310 is supplied through capacitor 311 tonode 312. The anode of diode 313 is coupled to node 312. The cathode ofdiode 313 is coupled to ground. Node 312 is connected through resistor314 to line 302. Also, node 312 is connected through resistor 315 to theemitter of a high-speed transistor 316. The base of high-speedtransistor 316 is connected through inductor 317 to ground. Collector oftransistor 316 is connected to the transmit antenna 318. Also, thecollector is coupled through inductor 319 to node 320. Node 320 iscoupled through resistor 321 to line 302 and across capacitor 322 toground. The transmit antenna 318 is connected through resistor 323 andcapacitor 324 to ground. Thus, the transmitter generates a short burstof radio frequency energy at the transmit antenna 318. The antenna maybe a vertically polarized antenna, a circularly polarized antenna,antennae based on cross-dipoles or other implementations known in thearts. Also, the antenna may be dithered or otherwise maneuvered toimprove sensitivity of the receiver.

The use of the pulsed RF system has a very low average power, complyingwith FCC Part 15 regulations.

FIG. 6 illustrates a receiver for use with the system of the presentinvention. The receiver includes a receive antenna 350 and a singlediode sample gate based on diode 351. The cathode of diode 351 isconnected to the receive antenna 350. Also, the strobe signal from line242 of FIG. 4 is supplied through capacitor 352 to the cathode of diode351. Resistor 353 is coupled from the cathode of diode 351 to ground.The anode of diode 351 is coupled to node 354. Capacitor 355 holds thesampled voltage between node 354 and ground. A resistor 356 is coupledfrom node 354 to the positive five-volt supply. Node 354 is connectedthrough capacitor 356 to the base of transistor 357. The emitter oftransistor 357 is connected to ground. A resistor 358 is connected infeedback from the collector of transistor 357 to its base. The collectorof transistor 357 is connected through capacitor 398 to a sequence ofaudio amplifiers beginning with inverter 359, having resistor 360 infeedback. The output of inverter 359 is connected through capacitor 361and resistor 362 to the input of inverter 363. Inverter 363 has resistor364 and capacitor 365 connected in parallel in feedback from the outputto the input. Also, the output of inverter 363 is connected throughcapacitor 366 and resistor 367 of the input of inverter 368. Inverter368 has resistor 369 and capacitor 370 connected in feedback inparallel. The output of inverter 368 is an equivalent time sample signalon line 371. This signal is supplied across resistor 372 to the input ofinverter 373. The output of inverter 373 is supplied to the anode ofdiode 374. The cathode of diode 374 is connected to node 375, which isconnected across capacitor 376 to ground. Also, a resistor 377 isconnected from node 375 to the input of inverter 373. Node 375 isconnected through resistor 378 to the input of inverter 379. Inverter379 has capacitor 380 in feedback and its input is connected acrossresistor 381 to ground. The output of inverter 379 is connected throughresistor 382 to the collector of transistor 357 and provides automaticgain control for the amplifier sequence in response to the voltagegenerated on capacitor 376.

The equivalent time signal on line 371 is also connected throughcapacitor 383 and resistor 384 to node 385. Node 385 is connected acrosscapacitor 386 to ground and resistor 387 to ground. It is also connectedto a video output connector 388 for connection to an analyzing circuit.

The signal on line 371 is supplied as the equivalent time signal RX tothe threshold detector shown in FIG. 4.

The values of the resistors and capacitors are illustrated in FIGS. 4through 6 for the example circuit shown. The NAND gates are 74HCOO andthe inverters are 74AC04, except in the receiver in which the invertersare implemented using MC14069UB inverters.

The sample circuit uses a single-ended, single diode sample gate, whichoperates with low power and high efficiency for sampling the smallsignals at the fast rate required by the present invention. Otherreceiver topologies might be used, such as those described in myco-pending U.S. patent application entitled Ultra-Wide Band Receiver,Application Ser. No. 08/044,745, filed Apr. 12, 1993, owned at the timeof invention and currently by the same assignee as the presentinvention. Such application is incorporated by reference in order toteach alternative receiver topologies.

FIG. 7 illustrates a simple head position sensing system implementedaccording to the present invention. In this system, a transmitter 500 ismounted on a user's headset 501, worn by a user of a computer system502. The receiver box 503 is mounted on the computer system 502 andconnected across cable 504 to a standard mouse interface. The receiverbox 503 includes a first receiver 505, a second receiver 506 and a thirdreceiver 507 each generating a time-of-flight measurement for pulsesgenerated by the transmitter 501. The receiver box 503 produces dataindicating the time-of-flight from the transmitter 500 to each of thethree receivers 505, 506, 507 can be used for precise position detectionof the transmitter 500 mounted on the headset 501. The user is tetheredby a small diameter coaxial cable 508 to the receiver box 503 to providetiming in the embodiments described. Computer system 502 includes thestandard monitor 510 and keyboard 511 and may be used for executinginteractive computer programming based on the position data producedaccording to the present invention. Various arrangements of thetransmitters and receivers may be used to triangulate, providing sixaxis information: x, y, z in translation and 3 axes of rotation for thetransmitter 500.

Accordingly, a very high resolution position sensing system has beenprovided based on direct time-of-flight measurement of radio frequencypulses. The system is simple and highly accurate, greatly improving overprior systems for providing this type of information.

The system is capable of providing submillimeter resolution made withcomponents costing less than about $10.00. The invention can be appliedto interactive media systems, robotics, automotive occupant positionsensing, digital surgery, and a wide variety of other applications wherehigh resolution position sensing is desired.

The foregoing description of preferred embodiments of the invention hasbeen presented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formsdisclosed. Obviously, many modifications and variations will be apparentto practitioners skilled in this art. It is intended that the scope ofthe invention be defined by the following claims and their equivalents.

What is claimed is:
 1. An apparatus for measuring time-of-flight of anelectromagnetic pulse, comprising:a transmitter which transmits asequence of electro-magnetic pulses in response to a transmit timingsignal; a receiver which samples the sequence of electro-magnetic pulseswith controlled timing, in response to a receive timing signal, andgenerates a sample signal in response to the samples; a timing circuitwhich supplies the transmit timing signal to the transmitter andsupplies the receive timing signal to the receiver, the receive timingsignal causing the receiver to sample the sequence of electro-magneticpulses such that time between transmission of pulses in the sequence andsampling by the receiver sweeps over a range of delays; and a sampledetection circuit which in response to the sample signal and the timingcircuit indicates a time-of-flight between the transmitter and thereceiver of pulses in the sequence.
 2. The apparatus of claim 1, whereinthe delays in the range of delays over which the timing circuit sweepsthe time between transmission of pulses in the sequence and sampling bythe receiver vary by less than 100 nanoseconds.
 3. The apparatus ofclaim 2, wherein delays in the range of delays vary by less than 10nanoseconds.
 4. The apparatus of claim 1, wherein the transmit timingsignal causes the transmitter to transmit the sequence ofelectro-magnetic pulses at a consistent pulse repetition rate to definea time between pulses, and the time between pulses is greater than thedifference in the delays at the beginning and the end of the range ofdelays.
 5. The apparatus of claim 1, wherein the transmit timing signalcauses the transmitter to transmit the sequence of electro-magneticpulses at a pulse repetition rate, and wherein the receive timing signalsweeps over the range of delays in a sweep cycle such that pulses in thesequence are sampled at the pulse repetition rate and with differentdelays in the range of delays, such that the sample signal representsmagnitude of a received pulse in the equivalent time.
 6. The apparatusof claim 5, wherein the receiver includes automatic gain controlcircuitry to control magnitude of the sample signal in equivalent time.7. The apparatus of claim 5, wherein the sweep cycle is repeated at asweep rate of less than 16 kilohertz.
 8. The apparatus of claim 5,wherein the sweep cycle is repeated at a sweep rate of less than 100Hertz.
 9. The apparatus of claim 7, wherein the pulse repetition rate isgreater than about 1 megahertz.
 10. An apparatus for detecting positionof an object, comprising:a transmitter, for placement on the object,which transmits a sequence of electro-magnetic pulses in response to atransmit timing signal; a receiver which samples the sequence ofelectro-magnetic pulses with controlled timing, in response to a receivetiming signal, and generates a sample signal in response to the samples;a timing circuit which supplies the transmit timing signal to thetransmitter and supplies the receive timing signal to the receiver, thereceive timing signal causing the receiver to sample the sequence ofelectro-magnetic pulses such that time between transmission of pulses inthe sequence by the transmitter and sampling by the receiver sweeps overa range of delays; and a pulse detection circuit which in response tothe sample signal and the timing circuit generates a pulse detect signalwhen the sample signal reaches a threshold during a sweep; and a signalprocessor, coupled with the pulse detection circuit, to indicate aposition of the object in response to the pulse detect signal.
 11. Theapparatus of claim 10, wherein the pulse detect signal indicates receiptof a transmitted pulse propagated directly from the transmitter.
 12. Theapparatus of claim 10, wherein the timing circuit includes:a pulse rateclock; a cable, having cable delay time, connecting the pulse rate clockto the transmitter; a circuit located at the transmitter whichtranslates the pulse rate clock from the cable into the transmit timingsignal; and a controlled delay circuit, coupled with the pulse rateclock, which produces the receive timing signal at the receiver inresponse to the pulse rate clock delayed in time to compensate for thecable delay.
 13. The apparatus of claim 10, wherein the timing circuitincludes an oscillator producing a pulse clock, a circuit whichtransmits the pulse clock to the transmitter with a known propagationdelay as the transmit timing signal, and a controllable delay circuitreceiving the pulse clock and producing the receive timing signaldelayed in time by a controllable amount in a sweep over a rangebeginning near a time corresponding to the known propagation delay andending near a maximum predicted range of time-of-flight for anelectromagnetic pulse from the transmitter to the receiver.
 14. Theapparatus of claim 13, wherein the signal processor comprises a counterwhich counts at a count rate from a beginning of the sweep untilgeneration of the pulse detect signal to generate a counter outputindicating a position of the transmitter.
 15. The apparatus of claim 13,wherein the controllable delay circuit includes a voltage controlleddelay circuit having a control input, and a voltage ramp generatorcoupled to the control input of the voltage controlled delay circuit tosweep the receive timing signal.
 16. The apparatus of claim 15, whereinthe voltage ramp generator comprises a digital to analog converter. 17.The apparatus of claim 15, wherein the voltage ramp generator comprisesan analog exponential ramp generator, and the voltage controlled delaycircuit produces a delay which is an exponential function of voltage onthe control input.
 18. The apparatus of claim 10, wherein the receiverincludes an automatic gain control circuit.
 19. The apparatus of claim10, wherein the transmit timing signal causes the transmitter totransmit the sequence of electro-magnetic pulses at a pulse repetitionrate, and wherein the receive timing signal sweeps over the range ofdelays in a sweep cycle such that pulses in the sequence are sampled atthe pulse repetition rate and with different delays in the range ofdelays, such that the sample signal represents magnitude of a receivedpulse in the equivalent time.
 20. The apparatus of claim 19, wherein thereceiver includes automatic gain control circuitry to control magnitudeof the sample signal in equivalent time.
 21. The apparatus of claim 19,wherein the sweep cycle is repeated at a sweep rate of less than 16kilohertz.
 22. The apparatus of claim 19, wherein the sweep cycle isrepeated at a sweep rate of less than 100 Hertz.
 23. The apparatus ofclaim 21, wherein the pulse repetition rate is greater than about 1megahertz.
 24. The apparatus of claim 10, including at least oneadditional receiver spaced away from the other receiver, and coupled tothe timing circuit and the signal processor, which generates anadditional sample signal.
 25. An apparatus for detecting position of anobject, comprising:a transmitter, for placement on the object, whichtransmits a sequence of microwave pulses in response to a transmittiming signal with a pulse repetition rate greater than about 1megahertz; a receiver having an antenna, a sample gate coupled to theantenna, which samples which samples a signal on the antenna at thepulse repetition rate and with controlled timing in response to areceive timing signal, and an amplifier coupled to the sample gateincluding automatic gain control such that pulses in the sequence aresampled to generate an equivalent time sample signal; a timing circuitwhich supplies the transmit timing signal to the transmitter andsupplies the receive timing signal to the receiver, the receive timingsignal causing the receiver to sample the sequence of electro-magneticpulses such that time between transmission of pulses in the sequence bythe transmitter and sampling by the receiver sweeps over a range ofdelays varying by less than 20 nanoseconds in a sweep duration ofgreater than about 10 milliseconds; and a signal processor, coupled withthe receiver, to indicate a position of the object within a range ofless than 10 feet in response to the equivalent time sample signal. 26.The apparatus of claim 25, wherein the signal processor samples theequivalent time sample signal at a rate of greater than 1 megahertz. 27.The apparatus of claim 25, including at least one additional receiverspaced away from the other receiver, and coupled to the timing circuitand the signal processor, which generates an additional equivalent timesample signal.
 28. A method for detecting position of an object at arange of less than 10 feet, comprising:mounting a transmitter on theobject; transmitting from the transmitter a sequence of electromagneticpulses; detecting time-of-flight of the electromagnetic pulses from thetransmitter to a receiver by sampling the sequence of pulses withcontrolled timing to produce an equivalent time representation of atransmitted pulse at the receiver and processing the equivalent timesignal to indicate the time-of-flight; and processing the time-of-flightto indicate position of the object.
 29. The method of claim 28, whereinthe time-of-flight is less than 10 nanoseconds.
 30. The method of claim28, further including:synchronizing the transmitting and sampling stepsby supplying a reference clock to the transmitter across a timing cablehaving a predictable delay, and controlling the timing of the samplingstep in response to the reference clock and the predictable delay. 31.The method of claim 28 wherein the time between transmission of pulsesand sampling is swept over a range of delays.